With the development and expansion of the chemical industry, it has become increasingly desirable to provide equipment for the containment and transport of corrosive fluids, gases and slurries which are resistant to corrosion caused by such compounds. Particularly in the plating, primary metals, water purification, and chemical process industries it has been apparent that there is a distinct need for corrosion resistant pipe, pumps, fittings and the like. For this reason the industry has commonly used corrosion resistant but expensive materials such as "Hastelloy", tantalum and titanium alloys, stainless steels such as Type 316 and "Carpenter-20" alloy. Additionally, the industry has increasingly turned towards non-metallic materials such as plastics, specifically including fiberglass, for economic reasons, and has in some cases hybridised the two, e.g., by lining the inside of a metallic pump with a plastic material such as "Teflon".
However, in many applications it is not sufficient to simply substitute a plastic material for a metallic one, the design of the pump or other fitting otherwise remaining the same. This is because the strengths of the materials vary so greatly and in differing ways depending on the type of force exerted thereon.
A clear example of this is found in the centrifugal pump industry. Centrifugal pumps typically comprise essentially circular impellers of metallic or non-metallic materials dependent on the service conditions, having vanes on at least one surface thereof, driven by perpendicular shafts extending through the pump housing being sealed therefrom and driven by electric motors. For economic reasons it is preferable that the shaft not be made of corrosion resistant material; this requires that it be protected by a sleeve within the pump housing. Typically the electric motor is coupled to this shaft, which is supported by bearings in a bearing carrier assembly, and then threaded to the impeller which is contained within a housing of the well-known volute shape so as to provide maximum pumping efficiency.
It is possible that in the event of reverse motor connection or possibly back flow in the pipeline to which the pump is connected that the impeller might be inclined to come unthreaded from the motor shaft and back off it so as to impact the inside of the pump housing, thus causing damage to either the pump, the housing or both. A typical expedient for solving this problem in the prior metal pump art would be to lock the two together by means of, e.g., a cotter pin, a metal tab, set screw or the like. This does not eliminate the requirement of a fastener to hold the impeller fixed in the axial direction; addition of such an axial fastener, such as a nut, requires a seal between fastener and impeller to prevent corrosion of the pump shaft. Such seals, being made of imperfectly resilient materials, tend eventually to take a set and leak.
Replacement of all the metallic parts contacting the corrosive liquid or slurry to be pumped, as discussed above, involves more than a simple substitution of the desired plastic or fiberglass material for the metallic material. Each element of the pump design must be carefully considered to determine whether or not the lower strength fiberglass or plastic material will be suitable, or whether the design needs to be revised. In the case of the centrifugal pump, for example, the exterior casing forming the volute can be more or less directly transformed from metal to fiberglass and can be made by one of a number of well-known manufacturing processes. Similarly, the disk-like impeller can be molded in one piece construction from fiberglass or plastic. However, it will be appreciated by those skilled in the art that the cotter pin used to prevent unscrewing of the impeller from the motor shaft can not be so simply formed of fiberglass since the shear strength of fiberglass is very low. Nor, of course, can a simple metallic cotter pin be used because this would eventually corrode and fail to perform its function when required. Instead, new means must be found to prevent unscrewing of the impeller from the motor shaft so as to prevent the impeller from coming into contact with the interior of the pump housing and causing damage to both. Prior practices have included forming a keyway in the fiberglass impeller and mating this with a key on the motor shaft and using a second fiberglass part having a metallic nut molded therein for holding the impeller with respect to the shaft. This approach has several drawbacks. First, of course, is its complexity; it requires two fiberglass parts, where one would be more desirable, as well as a keyway. The key is still subject to corrosion and the threaded cap itself can, of course, come loose from the shaft since it cannot be held by the key.
One prior art method, shown in U.S. Pat. No. 365,263 to Leib, shows locking a threaded coupling agent unscrewing by making the mating members of similar external shape, e.g. octagonal, and proving a matching sleeve. The sleeve is then locked in axial position. However, this method is only suitable when the external surfaces happen to line up--in the octagonal case, only every 45.degree. of relative rotation. This is clearly undesirable for reasons of versatility in assembly, as well as manufacturing cost and convenience.
Therefore, a need exists in the art for a method of holding an impeller fixed with respect to a shaft which does not require metallic coupling members and in which all threadably connected parts are positively locked against unscrewing of their threads. It would be additionally desirable, of course, if such a construction could be found which did not involve substantial additional expense in the construction of the pump and which simplified its construction.